Phenylbutyrate is a potential tumor differentiating agent and cytoxic agent which could alter tumor growth and progression, slow or inhibit metastases, and/or effect response to other forms of therapy. It does not have the systemic toxicities of other differentiating agents. This study will determine the maximum tolerated dose of phenylbutyrate given orally three times a day to patients with refractory CNS tumors before evidence of tumor progression. The study will also determine the therapeutic activity of the study drug.